RFR: CODETOOLS-7903050: Add an FAQ entry for `javatest.maxOutputSize`

[Jonathan Gibbons]

There continues to be confusion about setting javatest.maxOutputSize. 

This changes adds an FAQ entry specific to this issue.

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Commit messages:
 - CODETOOLS-7903050: Add an FAQ entry for `javatest.maxOutputSize`

Changes: https://git.openjdk.java.net/jtreg/pull/37/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtreg&pr=37&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903050
  Stats: 11 lines in 1 file changed: 11 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jtreg/pull/37.diff
  Fetch: git fetch https://git.openjdk.java.net/jtreg pull/37/head:pull/37

PR: https://git.openjdk.java.net/jtreg/pull/37